A collet adapter can include a collet in which a workpiece can be clamped for machining. The collet is usually releasable from the collet adapter so that different sized collets can be mounted in the collet adapter. Different sized collets are used for machining different sized workpieces. Collets have longitudinal “fingers” that move radially between clamped and unclamped conditions and move between these positions by forward and backward movement of the collet relative to the collet adapter. Thus, forward movement of the collet within the collet adapter can allow the fingers to shift radially outwardly to open the collet to receive or release a workpiece (the unclamped condition), while rearward movement of the collet shifts the fingers radially inwardly about a workpiece to allow the collet to clamp a workpiece. Clamping and unclamping can also occur in the reverse direction. Once clamped, the collet adapter can be driven to rotate as required for machining of the portion of the workpiece that extends out of the collet.
In addition to clamping by the collet, a workpiece being machined can be supported by a workpiece support which is separate from the collet adapter. The workpiece support is spaced from the front end of the collet adapter and reacts or counters loads imposed on the workpiece by the machining process. The workpiece being machined can be clamped to the workpiece support during machining to enhance the stability with which the workpiece is supported, but the clamping load must be such that it allows the workpiece to be rotated about the centre-line of the workpiece relative to the workpiece support.
FIG. 1 illustrates a general arrangement showing a collet adapter 10, and includes a collet holder 11 and a collet 12 mounted within the collet holder 11. In practice, a workpiece W is mounted and gripped within the collet 12 and a portion of the workpiece projects as shown from the collet 12 for machining. The collet adapter 10 is mounted in practice within the headstock of a CNC machine (not shown) and is rotatable via a drive arrangement of the headstock for rotating the workpiece W. The workpiece W would also be supported in practice on a workpiece support 13 which has a V groove 14 for accepting the workpiece. A clamp C can be brought down into contact with the workpiece W to restrict movement of the workpiece W within the support 13 other than rotational movement about the workpiece centre-line or axial movement along the workpiece centre-line.
Each of the collet adapter 10 (through the collet 12) and the workpiece support 13, clamp the workpiece W for machining. Thus, where the axes of the collet adapter and the workpiece support are not aligned, the workpiece is subject to misalignment loading during machining. Any misalignment between the workpiece support and the collet adapter can cause inaccuracy in the position of the workpiece on the workpiece support and that can affect the accuracy with which the workpiece is machined. It must be appreciated that in machinery of the kind in which the invention is to be employed, accuracy is required at micron level, so that only minor inaccuracy can have a significant effect on the production of accurate machined workpieces. A problem with prior art machinery arises in arranging acceptable alignment between the respective axes of the collet adapter and the workpiece support, which is too costly and complex.
The present applicant has secured U.S. Pat. No. 6,598,884 that discloses a collet adapter which has fine adjustment provided by a series of adjustment pins that directly engage the collet at circumferentially spaced locations to adjust the angle of the central longitudinal axis of the collet relative to the central longitudinal axis of the collet adaptor. While a collet adapter of this kind can be finely adjusted to reduce misalignment errors, the manual nature of the adjustment means that set-up time is increased while the adjustment is made.
U.S. Pat. No. 4,776,734 discloses a tool holder with two adjustable components which are secured together by set screws and adjustment is made via a ring and grub screw arrangement to fix relative eccentricity between the two components. Again, the disclosure is of a manual adjustment mechanism that is adjusted during machine set-up.
FIGS. 2i and 2ii illustrate different forms of misalignment of a workpiece W that can occur within prior art machinery and which the present invention is intended to correct or accommodate. Both figures show a portion of the collet adapter 10 including a collet 12 and a workpiece 15. The collet 12 is clamped within the collet adapter 10 and the workpiece 15 projects from the collet 12. The misalignment in each figure is greatly exaggerated for the purposes of explanation.
In FIG. 2i, the same workpiece 15 is shown in four different positions about the axis of rotation of the collet adapter 10 to show the position of the workpiece as the collet adapter 10 rotates. From this figure, it can be seen that the axis A of the workpiece 15 is at an angle to the axis B of rotation of the collet adapter 10. Misalignment of this kind occurs as a result of axial errors caused in the manufacture of the collet adapter and the collet and associated components.
In FIG. 2ii, the same workpiece 15 is again shown in four different positions about the axis of rotation of the collet adapter 10 to show the position of the workpiece as the collet adapter 10 rotates, but in FIG. 2ii, the type of misalignment shown is different to that shown in FIG. 2i. In FIG. 2ii, the axis C of the workpiece 15 is parallel to, but slightly offset from the axis B of rotation of the collet adapter 10. Misalignment of this kind occurs as a result of radial errors caused in the manufacture of the collet adapter and the collet and associated components.
Each of the forms of misalignment shown in FIGS. 2i and 2ii can result in unacceptable machining errors in a machined workpiece. Accordingly, it is an object of the present invention to overcome or at least alleviate one or more of the problems associated with the prior art.